I honestly think that RAID is fairly cheap compared to other solutions - especially in the long run as you don't physically need to continue to back up to new disks each time - or overwrite old ones. That said, I would still back up valuable data on a RAID array.
The problem with RAID is that it is not the easy to migrate from your current solution - and even harder to go back.
I would suggest buying another disk of the same capacity - preferrably from the same manufacturer. If you have a newer chipset - like the Intel set with Southbridge ICH6R (most 915/925 boards), you can create a RAID array with Intel Matrix from two disks. You can create a RAID 0 partition for the operating system and apps which will give you a little speed boost and a RAID 1 partition for valuable data back-up. If one drive fails, you lose the RAID o partition, but you can reconstruct - or get back the other one. AMD board also support similar arrangements.
If you have 2 80GB drives, you can have 100GB RAID0 and 30GB RAID1 (as this partition is duplicated across the disks) = 160GB. Or you can configure it how you want remembering to halve the storage of the RAID 1 partition.